1. Field of the Invention
The present invention relates to a display device and a method of fabricating a display device, and more particularly, to a backlight device of a liquid crystal display (LCD) and a method of fabricating a backlight device of a liquid crystal display.
2. Discussion of the Related Art
Cathode ray tubes (CRTs) have been commonly used as monitors for televisions, measuring devices, and information terminals. However, one problem of CRTs is their size and weight. Accordingly, display devices, such as liquid crystal display (LCD) devices that make use an electro-optics effect, plasma display panel (PDP) devices that make use of a gas discharge, and electro-luminescence display (ELD) devices that make use of an electro-luminescence effect, have been developed as replacements for the CRTs.
Among these display devices, the LCD devices appear most promising because of their low power consumption, thin profile, and light weight, and are currently employed as monitors for desktop and laptop computers and as large-sized display devices. The LCD devices include an LCD panel for displaying picture images, and a driving part for supplying a driving signal to the LCD panel. The LCD panel has first and second glass substrates bonded to each other to have a predetermined interval, and a liquid crystal material layer is injected between the first and second glass substrates.
On the first glass substrate (i.e., a TFT array substrate), there are a plurality of gate lines arranged along a first direction at fixed intervals, a plurality of data lines arranged along a second direction perpendicular to the gate lines at fixed intervals, a plurality of pixel electrodes in respective pixel regions defined by intersections of the gate and data lines in a matrix-type configuration, and a plurality of thin film transistors (TFTs) responsive to signals transmitted on the gate lines for supplying signals transmitted on the data lines to the pixel electrodes.
The second glass substrate (i.e., a color filter substrate) has a black matrix layer for shielding light from areas excluding the pixel regions, a color filter layer for displaying colored images, and a common electrode for implementing the picture images.
The predetermined interval between the first and second glass substrates is maintained by a plurality of spacers, and the first and second glass substrates are bonded by a sealant pattern having a liquid crystal injection inlet. Once the first and second substrates are bonded together using the sealant pattern, a liquid crystal material is injected through the liquid crystal injection inlet into the predetermined interval. Since the LCD device controls transmittance of ambient light to display image data (i.e., pictures), an additional light source is required, such as a backlight device. The backlight device is classified into a direct-type device and an edge-type device according to a position of a lamp unit.
Presently, various different types of light source devices, such as electroluminescence (EL) devices, light emitting diode (LED) devices, cold cathode fluorescent lamp (CCFL) devices, and hot cathode fluorescent lamp (HCFL) devices, are commonly used as a backlight device. Among these different types, the CCFL devices have long lifetimes, low power consumption, and thin profiles, and are commonly used as the light source for a large-sized color TFT LCD devices.
In CCFL devices, a fluorescent discharge tube is implemented for making use of the Penning effect, which is caused by injection of a hydrargyrum gas containing Argon Ar and Neon Ne at a low pressure. In addition, electrodes are formed at both ends of the fluorescent discharge tube, wherein a cathode electrode is formed having a plate-shape. When a voltage is applied to the electrodes, electric charges inside the fluorescent discharge tube collide against the plate-shaped cathode during a sputtering state, thereby generating secondary electrons. Thus, circumferential elements are excited by the secondary electrons, whereby a plasma is generated. In addition, the circumferential elements emit strong ultraviolet rays, wherein the ultraviolet rays excite a fluorescent substance, thereby emitting visible light.
In the edge-type device, a lamp unit is formed at one side of a light-guiding plate, and includes a lamp, a lamp holder, and a lamp reflecting plate. The lamp emitting light is inserted into both sides of the lamp holder, whereby the lamp is protected from external impact. In addition, the lamp reflecting plate covers a circumferential surface of the lamp, and one side of the lamp reflecting plate is inserted to one side of the light-guiding plate to reflect the light emitted from the lamp to the light-guiding plate. In general, the edge-type device is implemented in relatively small-sized LCD devices, such as monitors for laptop and desktop computers. The edge-type device provide for uniform luminance, maintaining a long lifetime, and thin profile.
In general, a multi-color LCD device includes an LCD panel, a backlight, and a color filter. The multi-color LCD device uses a backlight device with a fluorescent lamp that produces three wavelengths as a light source. A white light emitted from the backlight device is divided into red, green, and blue colors in the color filter, and the divided colors are re-mixed to display various colors. The colors of the light source are determined according to chromaticity coordinates of the Commission International De L'eclairage (C.I.E.). That is, tristimulus values X, Y, and Z are calculated from a spectrum of a predetermined light source, and then x, y and z chromaticity coordinates of red, green, and blue are calculated according to a conversion matrix. Subsequently, x and y values of the red, green, and blue are expressed as rectangular coordinates, so that U-shaped spectral locus is drawn, which is called a CIE chromaticity diagram. The general light source has the chromaticity coordinates inside the U-shaped spectral locus. At this time, a triangular space of the red, green, and blue chromaticity coordinates becomes a color realization space. As the triangular space increases, the color realization ratio increases. The color realization depends on color purity and luminance. As the color purity and the luminance increases, the color realization increases. The tristimulus values X, Y, and Z indicate weight of a color-matching function approaching to one spectrum. For example, the Y tristimulus values is a stimulus value of the brightness.
Color temperature represents a hue of a white color according to a color change of the light emitted by a temperature of a heat source. On a common display monitor, the color temperatures are 9300K, 6500K, and 5000K. As the color temperature approaches 9000K, the hue of the white color contains a blue color. Similarly, when the color temperature is 6500K, the hue of the white color contains a red color, and when the color temperature is 5000K, a neutral hue is generated. The color temperature is obtained from the chromaticity coordinates (x, y) of the white color, wherein as the color temperature approaches 9000K it satisfies European broadcasting union (EBU) standards.
In an LCD device, a luminous spectrum of the backlight device is coupled with the color-matching function and transmission spectrum of the color filter to determine the tristimulus values at each wavelength of the visible light region. In order to obtain the various colors, controlling of a correlation between the backlight device and color filter and the tristimulus values is necessary. For example, the luminous spectrum of the backlight device has to be controlled to optimize the color realization and the color temperature, and the transmission spectrum of the color filter has to be controlled to optimize luminosity.
To create the white light, it is necessary to simultaneously use red, green, and blue light emitting diodes (LEDs). However, simultaneous use of the red, green, and blue LEDs to create the white color light may have limited application for practical use. Accordingly, one LED emitting the three wavelengths at a predetermined intensity to generate the white light is needed. Thus, it is required to develop the backlight device using a cold cathode fluorescent lamp for portable devices, such as notebook computers to generate quality color realization, and surface mount devices (SMD) LED of a hand-phone for low power consumption and miniaturization.
FIG. 1 is a cross sectional view of a backlight assembly according to the related art. In FIG. 1, the backlight assembly includes a fluorescent lamp 1, a light-guiding plate 2, a light-diffusion substance 3, a reflecting plate 4, a light-diffusion plate 5, and a prism sheet 6. When a voltage is supplied to the fluorescent lamp 1, some electrons remaining in the fluorescent lamp 1 migrate to the anode, and remaining electrons collide with molecules of argon Ar gas to excite the argon Ar molecules. Accordingly, positive ions are generated that collide with the cathode to generate secondary electrons. When the secondary electrons are discharged to the fluorescent lamp 1, the flow of the electrons collides with hydrargyrum vapor and becomes ionized, thereby emitting both ultraviolet and visible light. Then, the emitted ultraviolet light excites a fluorescent substance deposited on an interior of the fluorescent lamp, thereby emitting light.
Subsequently, the light-guiding plate 2 is formed of Poly Methyl Meth Acrylate (PMMA) having a high light transmittance, and causes the light emitted from the fluorescent lamp 1 to be a plate-type light source. An amount of the light transmitted by the light-guiding plate 2 is related by a ratio of the light-guiding plate thickness and the fluorescent lamp diameter, a distance between the light-guiding plate and the fluorescent lamp 1, and the shape of the reflecting plate. In general, the fluorescent lamp 1 is positioned along an incline at a center of the light-guiding plate 2 along the thickness direction, thereby improving transmission efficiency of the light. The light-guiding plate 2 may be used as a backlight device of an LCD device, and may be categorized as a printing-type light-guiding plate, a V-cut type light-guiding plate, and a scattering-type light-guiding plate.
The light-diffusion substance 3 is comprised of SiO2 particles, PMMA, and a solvent. Since the SiO2 particles have a porosity, they are used for diffusing the light. In addition, PMMA is used for bonding the SiO2 particles to a lower surface of the light guiding plate 2. The light-diffusion substance 3 is deposited on the lower surface of the light-guiding plate 2 in a dotted pattern, wherein the sizes of the dotted pattern gradually increase to obtain a uniform plate-type light source on an upper surface of the light-guiding plate 2. For example, the dotted pattern has a small size in a unit area near the fluorescent lamp 1, and the dotted pattern has a large size in a unit area apart from the fluorescent lamp 1.
The reflecting plate 4 is formed at the rear of the light-guiding plate 2, whereby the light emitted from the fluorescent lamp 1 is incident on the inside of the light-guiding plate 2. In addition, the light-diffusion plate 5 is formed on the upper surface of the light-guiding plate 2 upon which the dotted pattern is deposited, thereby obtaining a uniform luminance at different viewing angles. The light-diffusion plate 5 is formed of PET or poly-carbonate (PC) resin, and a particle-coating layer is formed on the light-diffusion plate 5 for diffusing the light.
The prism sheet 6 is formed to improve the front luminance of the light transmitted and reflected to the upper side of the light-diffusion plate 5. For example, the prism sheet 6 transmits the light of a predetermined angle, and the light incident on the other angles is totally reflected, whereby the light is reflected to the lower side of the prism sheet 6 by the reflecting plate 4 formed on the lower side of the light-guiding plate 2. The backlight assembly having the aforementioned structure is fixed to a mold frame, and a display unit disposed at an upper side of the backlight assembly is protected by a top sash. In addition, the backlight assembly and the display unit are received between the top sash and the mold frame being coupled to each other.
FIG. 2 is a cross sectional view of a backlight device using a fluorescent lamp according to the related art. In FIG. 2, the backlight unit includes a fluorescent lamp 11, a lamp housing 12, a light-guiding plate 13, a reflecting plate 14, a light-diffusion plate 15, a prism sheet 16, a protection sheet 17, and a main supporter 18. A fluorescent substance is coated on an interior of the fluorescent lamp 11 for emitting light, and the lamp housing 12 fixes the fluorescent lamp 11 and concentrates the light emitted from the fluorescent lamp 11 along one direction. The light-guiding plate 13 provides the light emitted from the fluorescent lamp 11 to an upper side of an LCD panel, and the reflecting plate 14 is bonded to a lower side of the light-guiding plate 13 to reflect the light leaking in an opposite side of the LCD panel to the light-guiding plate 13. The light-diffusion plate 15 is formed on an upper side of the light-guiding plate 13 to uniformly diffuse the light emitted from the light-guiding plate 13. In addition, the prism sheet 16 is formed on an upper side of the light-diffusion plate 15 to concentrate the light diffused in the light-diffusion plate 15 and to transmit the concentrated light to the LCD panel, and the protection sheet 17 is formed on an upper side of the prism sheet 16 to protect the prism sheet 16. The main supporter 18 receives and fixes the aforementioned elements.
In the aforementioned backlight device, the light emitted from the fluorescent lamp 11 is concentrated on an incident surface of the light-guiding plate 13, and then the concentrated light passes through the light-guiding plate 13, the light-diffusion plate 15 and the prism sheet 16, whereby the light is transmitted to the LCD panel. However, the backlight device using the fluorescent lamp has a low color realization ratio due to emitting characteristics of a light source. In addition, the backlight device does not have a high luminance due to limitations, such as size and capacity of the fluorescent lamp.
FIG. 3 is a cross sectional view of a backlight device using an LED according to the related art. In FIG. 3, LED light sources 22 are formed at both sides of a light-guiding plate 21 formed at a rear of an LCD panel to illuminate the LCD panel, so that it is possible to display images on a display screen in dark surroundings. The LED light source 22 is comprised of LED lamps 23 arranged along a one-dimensional structure in red, green, and blue order. The LED lamps 23 of the LED light source 22 are turned ON in order to display an image on the LCD panel. When a voltage is supplied to the red, green, and blue LED lamps 23, the three-colored LED lamps emit light that is scattered in the light-guiding plate 21, thereby generating a color mixture. As a result, the rear of the LCD panel is illuminated with white light.
FIG. 4 is a plan view of a backlight device using an LED according to the related art. In FIG. 4, the backlight unit includes LED lamps 23 and a light-guiding plate 21. The LED lamps 23 include red, green, and blue LED lamps 23a, 23b, and 23c, and the light-guiding plate 21 is formed at a rear of an LCD panel to uniformly diffuse the light emitted from the LED lamps 23 to the LCD panel. In order to emit white light using the LED lamps 23 as a light source, R, G, and B monochromatic light of the LED light source 22 (in FIG. 3) is emitted from the LED lamps 23. In a first area “a” of the light-guiding plate 21, a region 20 is created, wherein the different colored lights emitted from the respective LED lamps 23 do not overlap. Accordingly, it is not possible to create the uniform white light within the region 20. In a second area “b” of the light-guiding plate 21, R, G, and B monochromatic light emitted from the respective LED lamps 23 is mixed, thereby generating the uniform white light.
A luminous point is formed on the light-guiding plate 21 for effectively using the second area “b” of the light-guiding plate 21 in the backlight device, thereby using one-half of the light-guiding plate 21 spaced apart from the LED light source 22. By using the LED lamps 23 as the light source for illuminating the LCD panel, application in miniaturized and low power consumption devices, such as notebook-type personal computers, is possible. In addition, since the LED is a solid-state device, a DC voltage of 1.5V is supplied to the LED, whereby an AC-DC converter is not required. Accordingly, power consumption is greatly decreased. Furthermore, since the LED has greater reliability as compared to CRTs, the LED can be miniaturized and have a long lifetime.
However, using the backlight device in an LCD device is disadvantageous. For example, it is difficult to uniformly mix the red, green, and blue light emitted from the red, green, and blue LED lamps in order to emit the white light having a quality color realization ratio.